Correlation does not mean causation

A variety of images showing that correlation does not imply causation.

A plot showing the relationship between the number of Nobel prizes earned in each country, and chocolate consumption. Both data are adjusted by the population of each country (an indirect source of this graph here).

Phenomena like the shown in the image above usually come from the fact that both variables involved correlate highly with a third (not shown) variable. Many times this third variable is population (source: xkcd)

The fact that a linear association might be easily confounded with a causal relationship may be the reason of the mith that storks deliver babies. This article shows the strong correlation between stork population and human birth rate.

Conclusion, keep in mind that correlation does not mean causation. Although an unexpected correlation may lead to a serendipious discovering (source: xkcd)...

50 shades of grey

The readers of the novel series on unconventional desires may enjoy this little picture showing (literally) 50 shades of grey:

The picture is easily generated using the following R code:

shades <- seq(0, 1, length.out =50)
m.shades <- matrix(rep(shades, 50), 50, 50)
shades.of.grey <- grey(shades)
png("shades.png")
image(m.shades, col=shades.of.grey)
dev.off()

Estudio de un plan de operaciones para una nueva modalidad de handling en la terminal T1 del aeropuerto BCN-El Prat

Student

Àlvar Jaime Hileno

Supervisors

Oriol Lordan / Jose M Sallan

Defense date 

30th June, 2014

Abstract

Carrying the luggage to the airport on your way back home can be an exhausting and stressing task, and some customers of luxury hotels may be interested in a service in which their luggage is dropped directly in the hotel.

This project assesses the operational, legal and financial viability of a business consisting in facilitating the check in directly in the hotel, and carrying customer's luggage directly to the airport handling services in the context of the El-Prat Barcelona (BCN) airport. 

Estudio de implantación de un Pace-Maker en una línea de montaje de una empresa del sector de la automoción

Student

Mireia Magallón Gracia

Supervisors

Jose M Sallan

Defense date 

12th February, 2015

Abstract

English title: Study of the implementation of a pacemaker in an assembly line in the automotive industry

One of the tools that can be used in the context of lean manufacturing is a pacemaker. In this project several alternatives to implement a pacemaker in an assembly line are assessed. Then, the guidelines for implementing the selected alternative are detailed, and the implementation process is planned through the PMBOK standards in project management.

A list of linear programming solvers

Here is a list of software available to solve linear programming (LP) models, focused on free software with an R implementation. Most of this software is also able to solve integer linear programming (ILP) and mixed integer linear programming (MILP) models. The list is intended to be extended and more extensively annotated in the future. Don't hesitate to comment below if you see how this list can be extended or enhanced.

lp_solve

lp_solve is a MILP solver, that runs in a large variety of languages. Models can be passed via input files, an API (application programming interface) or an IDE (integrated development environment). It is also embedded into R via the lp_solve and lp_solveAPI packages. The reference guide is quite exhaustive, and a good place to start using this software.

lp_solve reference guide
lp_solve FAQ (MIT)
R package lp_solve
R package lp_solveAPI

GLPK

GLPK stands for GNU Linear Programming Kit. It is a set of callable libraries written in C intended to solve large scale LP, ILP and MILP models. It is developed by Andrei Makhorin, of the Moscow Aviation Institute. GLPK contains an standalone solver, glpsol, that can be called from the command line. It can read models written in a variety of languages, among them CPLEX, convenient for relatively small problems and GNU MathProg, a standard based on the AMPL format adequate to write large models with regular structure.

GLPK can be used in R via the Rglpk package. For windows users is available GUSEK, an IDE running the GLPK libraries.

GLPK homepage
R package Rglpk
GUSEK IDE

SYMPHONY

Symphony is an open-source solver of MILP problems written in C. It is an initiative of the COIN-OR project. It can read problems written in MPS and MathProg formats. It is also implemented in R through the Rsymphony package.

SYMPHONY homepage
SYMPHONY user's manual
R package Rsymphony

linprog

A library for solving linear programming models, quite popular in MATLAB. There is also a R package available. This library does not solve ILP/MILP models.

linprog in MATLAB
linprog R package

Análisis longitudinal de las consejerías cruzadas y su papel en la organización económica española

Citation

Poveda, G.; Sicilia, C.; Simo, P.; Sallan, J. M. (2015). Análisis longitudinal de las consejerías cruzadas y su papel en la organización económica española. Harvard-Deusto Business Research, 3(2):64-73.
http://dx.doi.org/10.3926/hdbr.7

Abstract

Interlocking directorates build relationships between corporations, and define the national economic organization. We analyze the corporate network of the IBEX 35 companies, plus two savings banks, and compare it with the same network in 2005. The most central positions in the network are occupied by large Spanish corporations, privatized in the 90s. On the other hand, in 2008 banks lose centrality, while savings bank’s centrality rises.

The sociological approach to economic action

Reading one classic paper by Mark Granovetter one sees how much we need the sociologist approach to economic action, far less naïve that the economists'. Two examples here:

To say, as Williamson does, that reliance "on internal promotion has affirmative incentive properties because workers can anticipate that differential talent and degrees of cooperativeness will be rewarded" (1975, p. 78) invokes an ideal type of promotion as reward-for-achievement that can readily be shown to have only limited correspondence for existing internal labor markets (...).
(Granovetter 1985:499)

(...) to avoid the the functionalims implicit in Williamson's assumption that whatever organizational form is most efficient will be the one observed. Before we can make this assumption, two further conditions must be satisfied: (i) well-defined and powerful selection pressures toward efficiency must be operating (...)
(Granovetter 1985:503)

References

Granovetter, M. (1985). Economic action and social structure: the problem of embeddedness. American Journal of Sociology, 91(3):481-510.
Williamson, O. (1975). Markets and Hierarchies. New York: The Free Press.